Silicon Bias Tab Gluing Procedure

Equipment Needed

• Grounding wrist strap
• Rohacell base for holding silicon
• Roahcell top for holding down silicon
• Square key to use as a weight
• Appropriate bias tabs, attached to a piece of fiberboard with tape
• Scalpel
• Fine tip tweezers
• Copper-clad G10 plates for holding assembly
• Conductive epoxy and mixing container for it
• Scale
• Sharpened dowel (end of handle of cotton swab)

1. Be sure you are wearing your grounding wrist strap.

2. Assembly should have a dummy plenum around the MCM. Place Rohacell base for holding silicon on the G10 plate. Clip the ZIF connector on the MCM output cable to the wire on the G10 plate. Lay the assembly on the Rohacell, strip side down (this requires flipping the assembly over). Cover the silicon with the appropriate (short or long) rohacell top, which leaves the corners to be glued uncovered. Place a square key on top of the rohacell to hold everything in place. Place the assembly on the G10 plate such that the kapton cable is extended flat.

3. Cut out two bias tabs of the appropriate flavor. The tabs come in groups of 10, which can be cut out in bulk and attached to fiberboard to make cutting single tabs easier. Also, it is easiest if the tabs are pre-tinned on one end for soldering to the kapton cable. Cutting the tabs is best done with a scalpel and using a square key as a straight edge guide. Cut just along the edge of the trace, and just beyond the exposed trace area on the ends. Cutting sometimes causes the tabs to curl, and it may be necessary to straighten them out, using tweezers.

4. Mix up conductive epoxy. It is advisable to use latex gloves when mixing the epoxy. We use Epotek epoxy, which requires a ratio of 10 parts silver paste to 1 part hardener (by weight). (Use an eye dropper to add the hardener). The workable pot life is ~1 hr, and the cure time is 6-8 hrs at room temperature. Typically, we allow it to cure overnight.

5. Now we will glue the first bias tab. Use a sharpened dowel to apply a dot of conductive epoxy to the silicon back plane. The dot should be in the lower (near the kapton cable) corner, around 4mm from each edge. Very little epoxy is needed--a dot of ~2mm diameter and fairly flat is sufficient. Be careful not to get any epoxy on the edge of the silicon detector. Pick up one bias tab with tweezers. We will glue the end which is NOT pre-tinned to the silicon. The tab should be aligned parallel to the edge of the Si and kapton cable, such that the pre-tinned end is near the appropriate pad on the kapton cable. After placing the exposed trace area into the conductive epoxy, carefully use the end of the tweezer to push down gently on the tab, to make sure it is well attached to the epoxy. If necessary, it is possible to rotate the tab to realign it with the soldering pad by pushing on the side of the tab with the tweezers.

6. Repeat the process for the second tab, and allow the assembly to cure overnight.